An electrophotographic image forming apparatus (e.g. printer) includes a fixing apparatus that fuses a toner image formed on a sheet of paper so as to fix the toner image on the sheet of paper. Known as an example of the fixing apparatus is a fixing apparatus which includes a pair of rollers, a fixing roller and a pressure roller, as shown in Patent documents 2 through 4.
The fixing roller is a roller member that has (a) a hollow shaft which is made of metal such as aluminum, (b) an elastic layer which is formed on the surface of the hollow shaft, and (c) a halogen lamp which is disposed as a heat source inside the shaft. A temperature control device controls a temperature of the fixing roller by performing on-off control of the halogen lamp in accordance with a signal outputted from a temperature sensor which is provided on the surface of the fixing roller.
Apart from the aforementioned halogen lamp, an external heating member which press-contacts the outer surface of the fixing roller is known as means for heating the fixing roller. Specific examples of the external heating member include an external heating roller which is disclosed in Patent document 1 mentioned below and an external heating belt which is disclosed in Patent documents 2 through 4. Since the external heating member directly contacts the outer surface of the fixing roller, the external heating member can heat the outer surface of the fixing roller more quickly than the aforementioned halogen lamp.
The pressure roller is a roller member which has a shaft and a heat-resistant elastic layer provided as a covering layer around the shaft. The heat-resistant elastic layer is made of a silicone rubber, for example. The pressure roller press-contacts the outer surface of the fixing roller, which causes elastic deformation of the elastic layer of the pressure roller. This forms a nip region between the fixing roller and the pressure roller.
In the above arrangement, fixing of an unfixed toner image on a sheet of paper is performed as follows. A sheet of paper having unfixed toner image formed thereon is caught in the nip region between the fixing roller and the pressure roller and transported by rotation of the fixing roller and the pressure roller. Then, the toner image formed on the sheet of paper is fused by heat given off from the outer surface of the fixing roller so that the toner image can be fixed on the sheet of paper.
In such a fixing apparatus, it is known that offset phenomenon such as cold offset and hot offset occurs when a temperature on the outer surface of the fixing roller falls outside an appropriate temperature range. The cold offset is a phenomenon in which due to lack of the amount of heat transferred to a sheet of paper, a part of insufficiently melted toner sticks to the fixing roller. The hot offset is a phenomenon in which due to overheating of a toner on a sheet of paper, a cohesive force of the toner decreases, and a part of the toner on the sheet of paper sticks to the fixing roller.
Thus, it is very important for the fixing apparatus to control a temperature of the fixing roller so that a temperature on the surface of the fixing roller is within the appropriate temperature range during sheet passing.
(Patent document 1)
Japanese Unexamined Patent Publication No. 038802/1999 (Tokukaihei 11-038802; published on Feb. 12, 1999)
(Patent document 2)
Japanese Unexamined Patent Publication No. 189427/2005 (Tokukai 2005-189427; published on Jul. 14, 2005)
(Patent document 3)
Japanese Unexamined Patent Publication No. 292714/2005 (Tokukai 2005-292714; published on Oct. 20, 2005)
(Patent document 4)
Japanese Unexamined Patent Publication No. 017031/1977 (Tokukaisho 52-017031; published on Feb. 8, 1977)
The appropriate temperature range of the outer surface of the fixing roller varies depending upon a sheet transport speed of an image forming apparatus where the fixing apparatus is installed. More specifically, the appropriate temperature range tends to shift to higher temperatures with increase of the sheet transport speed (process speed) and shifts to lower temperatures with decrease of the sheet transport speed. The reasons for the tendency are as follows. In cases where the sheet transport speed is high, a time for the contact between the sheet of paper and the outer surface of the fixing roller is short. Hence, sufficient heat is not transferred from the outer surface of the fixing roller to a sheet of paper unless a temperature on the outer surface of the fixing roller is relatively high. On the other hand, in cases where the sheet transport speed is low, the contact time is long. Hence, excessive heat is transferred from the outer surface of the fixing roller to a sheet of paper unless the temperature on the outer surface of the fixing roller is suppressed.
The so-called 4-cycle electrophotographic image forming apparatus is usually designed to have a substantially identical sheet transport speed in (a) cases where a color image is formed on a sheet of paper and (b) cases where a monochrome image is formed thereon (It is to be noted that an interval between the sheets of paper transported is different in the color image formation and monochrome image formation, and the number of sheets processed per unit time are therefore larger in the monochrome image formation.).
In a fixing apparatus included in the 4-cycle electrophotographic image forming apparatus, when the appropriate temperature range to fix color images is compared with the appropriate temperature range to fix monochrome images, a wide range of overlap therebetween exists, as illustrated in FIG. 10(a). Therefore, by setting a control value of a temperature of the surface of the fixing roller so as to fall within the overlap range, it becomes easy to perform temperature control such that a temperature of the outer surface of the fixing roller falls within an appropriate temperature range both in the case of color image fixing and the case of monochrome image fixing. This avoids the occurrence of the offset phenomena.
However, a demand for a four drum tandem engine image forming apparatus which is designed to have a higher sheet transport speed in forming a monochrome image on a sheet of paper than in forming a monochrome image thereon has recently been increasing (In other words, designed to have greatly different sheet transport speeds between in monochrome image forming mode and in color image forming mode.). This is because the image forming apparatus designed as above can increase the number of paper sheets processed for monochrome image without degrading image quality of a monochrome image formed.
In an image forming apparatus which is designed to have a higher sheet transport speed in forming a monochrome image on a sheet of paper than in forming a monochrome image thereon, when the appropriate temperature range to fix color images is compared with the appropriate temperature range to fix monochrome images, a very narrow range of overlap or no overlap therebetween exists, as illustrated in FIG. 10(b). In cases where a range of overlap therebetween is very narrow, it is difficult to perform temperature control for a surface temperature of the fixing roller falling within the overlap range even if a control value for a surface temperature of the fixing roller is set so as to fall within the overlap range. In this case, the cold offset problem and hot offset problem are likely to occur. In cases where no overlap range exists, it is difficult to suppress the offset phenomena no matter what control value is set.